PYRITE OXIDATION IN UNSATURATED SETTINGS

It is nothing new to say that pyrite oxidation, a principal agent in the production of acid mine drainage, results in significant negative impacts to water resources. This reaction has received, and continues to receive, considerable attention as we struggle to understand such a complex, infinitely interesting reaction that has such environmental significance.

The kinetics of pyrite oxidation have been well studied under saturated conditions, corresponding to pseudo-first order condition with respect to water, a required reactant. There have also been some measurements of the rate of reaction in vapor only conditions, which limit the activity of water. Even fewer are measurements of the rate of reaction for conditions in which water is present as both a vapor (humidity) and liquid. Inconsistent with the study of pyrite oxidation kinetics in saturated and vapor only conditions, pyrite oxidation associated with mineral extraction and many other field settings is dominated by unsaturated conditions where both liquid and vapor phase water are present.

A multiple linear regression of available literature data for vapor only oxidation of pyrite in an oxygen atmosphere produced the following rate expression

rate = 10^-5.7 e^-35600/RT RH^1.9 O₂^1.2

The reaction was determined to have an activation energy of about 36 kJ/mol, lower than the range of activations energies reported for the saturated oxidation of 50-80 kJ/mol.

The vapor regression rates for a range of relative humidity values have been combined with saturated rate law calculations and available measurements between 100% relative humidity and saturation to map the potential rates with variable water activity, at atmospheric oxygen concentrations. Rates are observed to range over two orders of magnitude. Applying the trend of rates to potential field conditions of mine waste rock, and inspection of literature, indicates an important influence of grain size and climate for regulation of rates of pyrite oxidation in waste rock.